Effect of Pre-Oxidation on Plasma Nitrided Steel AISI4140
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EFFECT OF PRE-OXIDATION ON PLASMA NITRIDED STEEL AISI4140 Lei Tang,1, 2 Xiaobing Zhao,1, 2 Jingcai Li,2 Kunxia Wei,1, 2, 3 and Jing Hu1, 2, 4 Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 44 – 49, March, 2020.
Steel AISI4140 is studied after plasma nitriding and preliminary oxidation. The structure of the preliminarily oxidized surfaces after the nitriding is studied by scanning electron microscopy (SEM). The free surface energy is computed from the results of the measurement of the angle of contact between a liquid drop and the surface studied. The thickness of the nitrided layer is determined. The phase transformations on the surface are considered. The effect of oxidation conducted prior to nitriding is shown to be positive. The optimum oxidation mode (300°C, 30 min) for improving the structure of the nitrided layer and more than doubling its thickness is chosen.
Key words: plasma nitriding, pre-oxidation, steel AISI4140, free surface energy, surface morphology.
been reported that pre-oxidation exerts a catalytic action on gas nitriding due to formation of a nanostructured surface layer [5, 6]. However, no data can be found on the effect of pre-oxidation on nitrogen steel AIS4140. The aim of the present work was to study the effect of pre-oxidizing of steel AISI4140 on acceleration of the process of subsequent plasma nitriding and to determine the characteristics of the surface layer obtained.
INTRODUCTION Rapid development of surface engineering technologies tightens the requirements on the properties of metallic alloys such as the friction, wear, corrosion and erosion resistances. Plasma nitriding is a very popular process used to raise the surface quality of gears from steel AISI4140. Unfortunately, the nitriding process conducted for increasing the thickness of the layer and the associated properties lasts for tens or even dozens of hours, which results in low efficiency of the process and high cost of the gears. This makes research aimed at raising the nitriding efficiency very important. It has been established that grain boundaries in nanostructured materials may be channels of rapid atomic diffusion and can reduce the activation energy of nitrogen diffusion and accelerate the nitriding kinetics [1, 2]. It has been shown in the last decade that such processes as surface mechanical attrition treatment (SMAT), wear under sliding friction and shot peening may create a nanostructure on the surface, which is significant for shortening the time and lowering the temperature of plasma nitriding [3, 4]. Pre-oxidation (before nitriding) changes the microstructure of the surface layer due to formation of oxides. It has 1 2 3 4
METHODS OF STUDY We studied steel AISI4140 (the Russian counterpart is 38KhMA) of the following chemical composition (in wt.%): 0.41 C, 0.91 Cr, 0.18 Mo, 0.83 Mn, 0.21 Si, 0.014 P, 0.011 P, the remainder Fe. The base material had a microhardness of about 400 HV0.05. Specimens 10 ´ 10 ´ 10 mm in size were cut from a used gear. All the s
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